1. Field of the Invention
This invention generally relates to pressure reduction systems and methods, particularly pressure reduction systems and methods utilizing static restriction devices such as orifices, venturi tubes, reduced diameter tubing or piping, and/or capillary tubes. Such systems and methods may be particularly useful to reduce relatively high stream pressures (e.g., greater than about 3200 p.s.i.g.) in aqueous supercritical waste oxidation reaction systems.
2. Description of the Related Art
Pressure reduction of fluid streams by at least about 500 psi has presented problems in the art. In particular, in supercritical water oxidation ("SCWO") systems, reactors typically operate at relatively high pressures (e.g., greater than about 3200 psig) and produce effluent streams which contain significant quantities (up to about 10 to 20%) of gas. The gas typically includes carbon dioxide and oxygen. When this relatively high pressure is reduced, the expansion of gas in the stream tends to create high velocities and/or severe cavitation in the pressure reduction system. As a result, erosion within the pressure reduction device tends to be significantly enhanced. In some circumstances, corrosion also tends to be enhanced.
In addition, many SCWO streams typically include inorganic solids which have not been oxidized. When the pressure of streams containing these solids is reduced, the solids tend to produce a highly erosive environment for components within the pressure reduction device.
The treatment of waste slurries and sludge such as municipal waste sludge or paper mill sludge by SCWO has been hampered as a result of the aforementioned erosion problems. Typically, SCWO units treating sludge or slurries must separate the solid inert material from the effluent prior to depressurization. Therefore, the separated solids must be removed by a batch system instead of a continuous system. Batch systems tend to be more expensive to operate than continuous systems.
Various systems and methods have been used to overcome relatively high pressure reduction erosion problems. In one such system, a throttling "control" valve is used. Throttling valves, however, tend to typically experience extremely high rates of wear on the throttling surfaces. These high rates of wear are due to the fact that the throttling surfaces tend to have a limited area available to absorb the kinetic energy produced by the pressure reduction.
Relatively high rates of wear are particularly problematic when throttling valves are used in systems with relatively low flow rates. In such systems, the annular region between the seat and trim (through which the fluid flows) tends to be very small. Thus, valve wear is experienced in a limited region, and is therefore accentuated.
Relatively high pressure reduction is problematic in systems with relatively low flow rates for other reasons. In particular, the relatively small flow passages in such systems tend to become clogged or plugged when even a relatively small amount of solid particles are present in the fluid stream. Whether eroded or plugged, in either case pressure reduction devices in relatively low flow systems tend to not provide their intended function. In addition, commercial pressure reduction valves designed for use with high pressure solids-containing slurries or sludge tend to be unavailable or unworkable at flow rates under about 5-7 gallons per minute ("gpm").
In some circumstances, the erosive effects of relatively large pressure reductions have been countered by making pressure reduction devices out of extremely hard substances such as tungsten, carbide, stellite, titanium nitride, or various ceramics. Even these hardened substances tend to experience unacceptably excessive and rapid wear under the high velocities caused by large pressure reductions. In addition, extremely hard materials tend to be brittle, thus making them unsafe and/or unreliable for normal service.
Throttling valves are by design nonstatic pressure reduction devices which vary their throughput as a function of a selected value such as pressure or flow rate. Because of the above-mentioned problems with such valves, practitioners in the art have attempted to reduce the amount of pressure reduction experienced by each valve in the system. Such attempts have resulted in multiple throttle valves being placed in series, with each valve experiencing a reduced amount of pressure reduction.
Multiple throttle valve systems, however, tend to be more expensive than single stage pressure letdown systems and tend to require numerous control loops. Moreover, to prevent the control loops from interacting, typically accumulators must be positioned between each valve. For significant pressure reduction systems there can be as many as two to ten valve systems in series, thereby greatly increasing the cost and complexity of the pressure reduction system.
In relatively low flow rate systems, multiple valve systems have the further drawback that the flow passages in each throttle valve will be reduced (as compared to single valve systems) since the total system pressure drop is distributed over all valves in the system. As a result, pressure control tends to be erratic since at least one of the valves may tend to plug if there are solids in the stream.
Another problem encountered in the art relates to the fact that density changes in typical SCWO systems are not necessarily proportional to pressure drop. These SCWO systems typically include significant proportions of carbon dioxide, and the pressure-density relationship for carbon dioxide differs significantly than that for ideal gases. It has been observed that significant density changes in the throttled effluent gases do not occur until pressure is reduced under about 800-1000 p.s.i.a. When pressure is reduced below about 800-1000 p.s.i.a., the stream velocity tends to increase dramatically due to a lowering of the effluent gas density. The bulk of the erosion tends to occur after the density decrease occurs. Proper placement and sizing of control valves to accommodate this density change tends to be difficult.
Practitioners have also attempted to use multiple port valves which contain several valve seats within a single valve body. Such multiple port valves, however, are difficult to use in lower flowrate systems (because of clogging problems), and tend to suffer from mechanical limitations at higher pressures.
Rather than use dynamic restriction devices to achieve pressure reductions, practitioners have also attempted to use static restriction devices such as orifice plates, reduced diameter pipes or capillary tubes, venturi tubes, or other static restriction pressure drop devices. These static restriction devices, however, only operate at one flow rate for a given pressure decrease, or, alternatively, at one pressure decrease for a given flow rate. Of course, these systems also experience wear due to erosion when significant high pressure drops are achieved. When such erosion occurs, the static pressure restriction device no longer provides the same pressure drop for a given flow rate. As a result, maintaining accurate pressure drop control within the system is difficult. And if pressure control is difficult, then control of other system parameters such as temperature, flowrate, reaction rate, etc. will also be difficult since these other system parameters tend to vary (at least indirectly) as a function of pressure. The problem of varying parameter control has thus hindered the use of static restriction devices for pressure reduction.
Static restriction devices also present problems in relatively low flow rate systems. In these systems, the size of the restriction in the orifice plate, reduced diameter pipe capillary tube, or venturi tube has to be small. As a result, these small restrictions tend to be highly susceptible to plugging.
U.S. Pat. No. 3,674,045 relates to a vortex valve fluid oscillator.
U.S. Pat. No. 1,725,782 relates to a method and apparatus for flow control.
U.S. Pat. No. 3,129,587 relates to a flow sensing device.
U.S. Pat. No. 4,887,628 relates to a fluidic apparatus in which a vortex amplifier functions as a choke valve to control flow in a flow line from, for example, a gas or oil well.
All of the above patents are hereby incorporated by reference.